A typical gas turbine engine contains an inlet, compressor, combustor, turbine, and exhaust duct. Air enters the inlet and passes through the compressor, with each successive stage of the compressor raising the pressure and temperature of the air. The compressed air mixes with fuel in the combustor and undergoes a chemical reaction to form hot combustion gases that pass through the turbine. The turbine, which contains a series of alternating stages of rotating blades and stationary vanes, is coupled to drive the compressor through a common rotor. As the hot combustion gases pass through the turbine, the thermal energy is converted into mechanical work by turning each stage of turbine blades that are contained within a disk, which is coupled to the rotor. The number of turbine blades forming each stage varies depending on location within the turbine and size of the turbine blades. Depending on the operating temperatures of the turbine, the turbine blades may or may not be cooled. Typically, the stages of the turbine closest to the combustor are cooled, with the aft most stages of the turbine uncooled.
Turbine blades are subject to both the elevated temperatures of hot gases exiting the combustor, as well as high mechanical stresses from rotational forces. A turbine blade that is exposed to each of these conditions for a prolonged time begins to creep or expand radially. Turbine blade creep is a result of plastic deformation occurring along the grain boundaries of the casting. When the thermal and mechanical loads on the turbine blade are released, the turbine blade cools and contracts. However, over time, complete contraction to the original grain structure does not occur and the deformation is permanent. A limited amount of permanent deformation is permissible before replacement of the turbine blade is required.
The creep rate can be reduced either by lowering the operating temperature or improving the resistance to creep. A common manner to accomplish the first option is to cool the turbine blades. However, cooling a turbine blade requires a more complex blade design that results in more costly manufacturing techniques. Furthermore, cooling a turbine blade requires using compressed air to cool the internal cavities of a turbine blade. This compressed air bypasses the combustion process and removes fluid that would drive the turbine, thereby reducing the overall efficiency of the turbine.
What is needed is a more cost effective means to reduce the creep rate of turbine blades, for both cooled and uncooled turbine blades, while not reducing the life of the blade attachment or turbine disk.